The background art of the invention is described taking an example of an excavator shown in FIG. 4. The excavator is provided with a crawler-type lower propelling body 1, an upper slewing body 2 loaded on the lower propelling body 1 so as to be capable of being slewed around an axis O vertical to the ground, a working attachment 3 attached to the upper slewing body 2, and a slewing drive unit as shown in FIG. 5 for slewing the upper slewing body 2. The excavator has a combination of a hydraulic unit incorporated with a hydraulic motor as a drive source and an electric unit incorporated with an electric motor as a drive source, as the slewing drive unit (see patent literature 1).
FIG. 5 shows a part of the excavator, the part relating to the slewing drive. The lower propelling body 1 has a propelling frame 4, and the upper slewing body 2 has an upper frame 5 mounted on the propelling frame 4 through a slew bearing 6. The slew bearing 6 includes an inner ring 6a and an outer ring 6b which are rotatable relatively to each other. The inner ring 6a is fixed to the propelling frame 4, and the outer ring 6b is fixed to the upper frame 5. The slewing drive unit U drives the upper frame 5 to slew it around the axis O.
The slewing drive unit U is constituted of a hydraulic unit U1 and an electric unit U2. The hydraulic unit U1 includes a hydraulic motor 7 as a drive source, a speed reducer 8 for reducing the rotational force of the hydraulic motor 7, and a pinion gear 9 mounted on an output shaft of the speed reducer 8. The pinion gear 9 meshes with an internal gear (slewing gear) 10 fixed to the inner ring 6a of the slew bearing 6 to transmit the rotational force of the hydraulic motor 7 to the upper frame 5. The electric unit U2 includes an electric motor 11 as a drive source, a speed reducer 12 for reducing the rotational force of the electric motor 11, and a pinion gear 13 attached to an output shaft of the speed reducer 12. The pinion gear 13 meshes with the internal gear 10 at a position circumferentially different from the position of the pinion gear 9 of the hydraulic unit U1. The electric motor 11 of the electric unit U2 is designed: to assist the hydraulic unit U1 when the upper slewing body 2 is driven to be slewed, that is, when the upper slewing body 2 is applied with a torque in the same direction as the slewing direction, for instance, when the upper slewing body 2 is accelerated or when the upper slewing body 2 is upwardly slewed while the slewing speed thereof is maintained; and to act as a power generator when the slewing is braked to charge regenerated electric power into an energy storage unit.
FIG. 6 shows a hydraulic circuit and an electric circuit relating to the units U1 and U2. These circuits include a hydraulic pump 14 to be driven by an engine 15 to thereby discharging hydraulic fluid, a slewing remote control valve 16 with a lever to be manipulated, and a pilot-type control valve 17 to be operated by the slewing remote control valve 16. The hydraulic fluid discharged from the hydraulic pump 14 is supplied to the hydraulic motor 7 through the control valve 17, thereby rotating the hydraulic motor 7.
Specifically, upon the manipulation on the slewing remote control valve 16 clockwise or counterclockwise for speed acceleration, that is, upon the manipulation on the slewing remote control valve 16 to the slewing drive side, the hydraulic motor 7 is rotated at a speed corresponding to the operation amount of the lever to thereby drive the upper slewing body 2 to slew it. On the other hand, upon the manipulation on the slewing remote control valve 16 to the neutral position side, a hydraulic brake valve not graphically shown is actuated to stop or decelerate the upper slewing body 2 to thereby brake the slewing operation of the upper slewing body 2. Furthermore, the circuits have a relief valve not graphically shown and positioned on the discharge side of the hydraulic pump 14, the relief valve to let redundant fluid generated when the upper slewing body 2 is accelerated or decelerated to a tank.
The circuits further include pilot pressure sensors 18 and 19 which detect a pilot pressure Pa to be outputted from the slewing remote control valve 16 in correspondence to the operation amount of the lever; motor pressure sensors 20 and 21 which detect pressures PA and PB at both sides of the hydraulic motor 7 for obtaining a slewing operation pressure (the slewing operation pressure: PA−PB); a controller 22; an energy storage unit 23 including a battery (secondary battery) and a capacitor (electrical double layer capacitor); and an inverter 24. Each of the sensors 18 through 21 generates a detection signal and inputs the detection signal into the controller 22. The energy storage unit 23 supplies electric power as a power source for the electric motor 11 thereto. The inverter 24 constitutes a control device in cooperation with the controller 22 to control a slew acceleration torque, a slew deceleration torque, and the like.
In the above circuits, the controller 22 performs: judging presence or absence of manipulation on the lever of the remote control valve 16 based on signals from the pilot pressure sensors 18 and 19; calculating a slewing direction, a slewing speed, a required acceleration torque or a required deceleration torque based on a slew operation pressure corresponding to a difference between the pilot pressures; determining whether the upper slewing body 2 is accelerated, fixed, or decelerated, and the controller 25 inputs a control signal into the inverter 24 based on a result of the determination. The inverter 24 issues a command to the electric motor 11 based on the control signal. Specifically, in the case where the required acceleration torque is greater than a predetermined set value, inputted to the electric motor 11 is a command for producing an acceleration torque for assisting the slewing operation. In the case where the slewing speed has reached a constant speed or a predetermined set speed, or in the case where the required acceleration torque is equal to or smaller than the predetermined set value, inputted to the electric motor 11 is a command for stopping the output of the acceleration torque and functioning as a power generator while being rotated by inertia force. Also upon the deceleration in or stopping of the slewing, a similar regeneration command is inputted. The electric motor 11 supplies the regenerated electric power to the energy storage unit 23 as charging electric power, in other words, charges the energy storage unit 23.
In the excavator, where the hydraulic unit U1 is assisted by the electric unit U2, it is possible to generate a necessary and sufficient torque by adjusting the part of torque to be assisted by the electric unit U2, while securing a maximum required torque as a whole. This produces advantage in the aspect of energy saving and prevention of hunting due to an excessive torque.
This type of construction machine, however, has the following drawbacks to be overcome. Since the electric unit U2 is required to output a high power, there should be used, as the energy storage unit 23 which is a power source for the electric unit U2, one having a relatively high value (several hundred voltages) of voltage between terminals. Hence, when the energy storage unit 23 is directly handled, i.e., at a non-ordinary work time, such as at the time of replacing the energy storage unit 23 or disposing the energy storage unit 23, the energy storage unit 23 is required to be discharged enough to lower the voltage between the terminals to a safe value, specifically, to a zero level or to a safe low voltage value, in order to secure the safety of workers.
As a technology relating to the above point, patent literature 2 discloses a hybrid construction machine having a generator motor which functions as a load (power generator) based on a discharging command to discharge the electric power from an energy storage unit. Similarly to this, it is possible, in the circuit shown in FIGS. 5 and 6, to discharge the energy storage unit 23 by electrically connecting it to the electric motor 11 so as to actually cause a slewing operation; however, because regenerative action occurs upon stopping the slewing operation, it is necessary to consume the electric power as heat by providing a resistor in a circuit for controlling the electric motor in order to prevent regenerative electric power of the electric motor from being accumulated in the energy storage unit 23. Furthermore, in the case of great heat resistance, cooling is required. These factors obstruct reduction in the cost and reduction in the facility space.
Patent Literature 1: JP 2005-290882A
Patent Literature 2: JP 2009-268222A